Syria suffered its worst terrorist attack since the start of the uprising when 55 people were reported killed and nearly 400 injured in twin car bomb blasts near a government intelligence building in Damascus.

Syrian officials and media blamed foreign-backed terrorist groups for the attack on Thursday, saying it was carried out "in the service of the interests of Israel and its allies in the region". Russia hinted that other unnamed countries were involved.

The opposition accused the regime of President Bashar al-Assad of carrying out the bombings to smear them as terrorists supported by foreign governments.

The death toll was the biggest in Damascus since the start of the uprising in March 2011. It was sophisticated and carefully co-ordinated. Witnesses said the first blast, at rush hour peak, was quickly followed by a second, larger one, maximising casualties, damage and terror. Most of the victims were reported to be civilians.

"Two booby-trapped cars loaded with more than 1,000kg of explosives and driven by suicide bombers carried out the terrorist blasts," said a statement by the interior ministry. The explosions left two large craters.

Syrian state TV showed bodies lying in the street amid smouldering vehicles. The official health ministry tally of the dead and 372 injured included a reference to "15 bags of body parts".

On Thursday night, it was reported that Syria's UN ambassador said there had also been an explosion in Aleppo. Bashar Ja'afari said the explosion on Thursday morning in Syria's second city "led also to … civilian victims and massive damages to private property". He gave no further details.

Experts compared the Damascus attacks to those on Iraqi government ministries in 2009, which were blamed by the authorities in Baghdad on groups based in Syria.

Thursday's blasts were condemned internationally. Kofi Annan, the special envoy for the UN and Arab League, said it was vital to implement his six-point plan for peace in Syria. Susan Rice, the US ambassador to the UN, said the bombings "remind us of the urgent need for a political solution in Syria before it is too late". The UN security council condemned the attacks.

The foreign secretary, William Hague, also condemned the blasts, adding: "Yet again it is the people of Syria who are suffering as a result of the repression and violence, which must come to an end."

Russia, a close ally of Assad, accused unspecified foreign countries of being behind the bombings. Saudi Arabia and Qatar have both called for the arming of anti-Assad rebels, and Gulf money is helping the Syrian opposition. The Russian foreign minister, Sergei Lavrov, said in Beijing: "Some of our foreign partners are doing practical things so that the situation in Syria explodes in a literal and figurative sense."

The question of who was behind the latest grim scenes of craters, blast-damaged buildings, burned-out vehicles, charred and mutilated corpses and body parts was being asked before the smoke had cleared in the al-Qazaz area, a busy intersection on the ring road in southern Damascus.

Haitham Maleh and Bassam Ja'ara, leading opposition spokesmen, blamed the regime and claimed it was reacting to earlier criticism from Annan about the failure to implement his plan. The Free Syrian Army (FSA), the main armed wing of the opposition, insisted it was observing a ceasefire under the terms of the Annan plan.

Syria's divided opposition has repeatedly accused the government of organising bombings to smear its enemies as terrorists and to deflect attention from the peaceful origins of the uprising and continuing popular support for it – though it does now include a significant armed element. Jabhat al-Nusra, a little known jihadi organisation, has claimed responsibility for previous bombings in Damascus, Hama and Aleppo.

Analysts note that Syria's intelligence services have a track record of dirty tricks, including assassinations and bombings in Lebanon and Iraq, and of manipulating Islamist groups for their own purposes.

One angry supporter of the regime described the bombings as the work of "Nato-backed FSA terrorists". Syrian TV showed a man pointing to the debris and saying: "Is this freedom? This is the work of the Saudis."

The opposition Revolution Leadership Council of Damascus reported security forces had shot and killed a 24-year-old man who was watching the scene of the explosion from his roof. It said mobile phone lines were cut off at the time of the blast and claimed that some bodies had no blood on them, suggesting they might have been brought to the scene by the regime.

It is possible that the escalation in violence may slow the already sluggish deployment of UN observers charged with monitoring implementation of the Annan plan: only 70 of a planned 300 are in the country.

Opinions are still divided about two attacks on similarly sensitive security installations in the Kafr Sousseh area of Damascus before Christmas. Opposition sources described mysterious transfers of prisoners to the heavily guarded buildings the day before the blasts, which coincided with the arrival of Arab League monitors. Western governments concluded that those attacks had been carried out by an al-Qaida-type group using military explosives and methods perfected in Iraq.

The next attack, a suicide bombing on 6 January in the nearby suburb of Meidan, was widely believed to have been staged by the Syrian authorities.

Film inadvertently broadcast on state media showed apparently dead or injured people moving around after being filmed, and objects such as shopping bags being placed at the scene by security personnel. State media arrived surprisingly quickly, adding to suspicions that it was an elaborate fake. Opposition sources claimed that names on the casualty lists in these incidents have been duplicated.